Does anybody know the Difference in Solidworks PDM API  Library,1.Interface,2.Enumerations and 3.Structure

You can consider interface as a module which provides some functionality. This is a programmatic representation of certain feature (for example vault can get file, file can be checked-in or checked-out etc.)

You can consider enumerator as a value representing certain constant option. This is used by interfaces and their methods (for example there different types of columns in the BOM: configuration, file path, name etc.)

You can consider structure as any additional information the methods of interface may need or may return (i.e. error details, permissions details, history record, file information)